The present invention relates to an apparatus for detecting the size of a document in a copier and others.
In a copier, for example, it is a common practice to detect the size of a document automatically so that a magnification may be set up based on the document size. Various approaches have heretofore been proposed for the detection of a document size such as sensing light which is reflected from a document, sensing interception of light which is caused by a document, and forcing a document which is laid on a platen to cut off a conduction circuit between the platen and a document cover.
The reflection type scheme mentioned above may be accomplished by forming projections and recesses in a document cover so as to cause light reflected by a document to be propagated through a path which differs from a portion where the document is present to a portion where it is absent, i.e., document cover portion, as disclosed in Japanese Unexamined Patent Publication (Kokai) No. 53-110837. Another prior art arrangement for the reflection type scheme uses the same number of photosensors as document sizes expected. Specifically, the photosensors are arranged below a platen along the width of a document and in those positions which do not interfere with imaging operations and are close to a reference line adapted for document setting. In this kind of arrangement, when a lamp adapted to illuminate a document is turned on, those sensors to which a reflection from a document is incident are turned on, but those to which it is not incident are turned off.
A problem with the system which relies on projections and recesses in a document cover is that an extra step is required for providing the document cover with projections and recesses. The photosensor type system, on the other hand, has a drawback that the same number of photosensors as the document sizes are required and, in addition, it fails to discriminate those documents which are the same in widthwise dimension although different in size, e.g. an A3 document placed vertically long and an A4 document placed horizontally long. While accurate discrimination between documents may be enhanced by increasing the number of photosensors, such brings about not only an increase in cost but also the need for a complicated mechanism for positioning the photosensors as well as troublesome adjustment. Even if the photosensors are mounted on a scanner which serves to illuminate a document, the same number of photosensors as the document sizes are needed to bring about the same problem and, in addition, to increase the number of wirings between the photosensors and a control unit which is mounted on a copier body.
Another problematic situation with the photosensor type system is that a document is sometimes laid aslant on a platen relative to a reference position and sometimes dislocated due to air pressure when a document cover is lowered to press a document. In such a situation, output signals of the photosensors are representative of erroneous data which prevent a document size from being determined with accuracy. Then, a copier having a capability of automatically selecting a paper size based on a document size would cause images to be reproduced on those papers which do not correspond to an actual document size.